NRL clubs have been told that links with sacked Essendon sports science guru Stephen Dank were the reason they were being investigated by the Australian Sports Anti-Doping Authority over the use of performance-enhancing drugs in the game.

Officials from the six NRL clubs named in a report by the Australian Crime Commission were briefed by ASADA representatives on Tuesday, and while they were told there are other aspects of the investigation, Dank is the common link between them all.

The controversial biochemist was employed by Manly between 2006 and 2010, and was involved with Cronulla for five months in 2011 but his links with the other four clubs were never previously identified.

Penrith, Newcastle, Canberra and North Queensland all say they have never had any association with Dank, but it is understood he told the ACC during two interviews last year that he had been a consultant to the Panthers and the Cowboys.

Both clubs say he was never involved with them, and his only association with Penrith was two meetings with former coach Matthew Elliott, who now coaches the Warriors, and head trainer Carl Jennings in 2011.

It is believed ASADA is investigating whether players at the Knights and Raiders who play for clubs where Dank was previously employed have maintained an association with him.

There is no suggestion of any wrongdoing by Dank or any of the players at the six clubs, who were all given severity ratings based on how deeply they were embroiled in the investigation.

Manly and Cronulla are understood to have received higher severity rankings than the other clubs as their association with Dank was more intense.

But both clubs are confident players will not be found to have taken performance-enhancing substances.

Sea Eagles officials were told that the investigation into their club goes back more than two years, while Sharks revealed that the ASADA probe was focused on the 2011 season.

''Yesterday representatives from the Cronulla Sharks attended a meeting chaired by Tony Whitlam, QC, the head of the NRL's integrity commission,'' the Sharks said on Wednesday.

''The meeting, which also involved NRL chief David Smith and representatives from five other NRL clubs, was to provide a better understanding of the investigation currently being undertaken by the ASADA.

''Following the initial meeting, club chairman Damian Irvine and general manager of football Darren Mooney then attended a private briefing with ASADA representatives.

''From those meetings, the Sharks can confirm that none of their players have tested positive to performance-enhancing substances and that ASADA are focusing their investigation on a short period of time in 2011.''

Dank has insisted that everything he did at Manly was above board, and is threatening to sue media outlets over allegations about his role at Essendon, which is under investigation from the AFL and ASADA for the use of peptides.

Meanwhile, North Queensland coach Neil Henry has slammed the handling of the investigation as ''embarrassing'', ''disgraceful'' and ''farcical''. ''The way the whole thing has been handled has been a disgrace,'' Henry said.

''To me, I think it's embarrassing for the NRL ... to be forced into a situation to defend ourselves ...''